(1) Field of the Invention
The present invention relates to a method for the production of a silicon nitride sintered body. More particularly, the present invention relates to a method for the production of a homogeneous silicon nitride sintered body having a high strength. Moreover, the present invention relates to a silicon nitride-containing composite nitride composition excellent in the sintering property.
(2) Description of the Prior Art
Since a silicon nitride sintered body is characterized in that most of linkages of atoms are covalent bonds, it is expected that the silicon nitride sintered body will be effectively used as a high-strength heat-resistant member, a highly corrosion-resistant member and a member showing a high strength at high temperatures.
As is well-known, in a silicon nitride sintered body, liquid phase sintering is effected by addition of a sintering aid to cause densification. As the sintering aid, there are used alkaline earth metal oxides such as MgO, rare earth metal oxides such as Y.sub.2 O.sub.3, and such oxides as Al.sub.2 O.sub.3. The sintering aid and silicon nitride powder are pulverized and mixed, and the resulting mixture is used as a starting material for a silicon nitride sintered body.
The oxide type additive reacts with silicon nitride or a film of SiO.sub.2 present on the surface of the crystal of silicon nitride powder to form a grain boundary phase, and even if the starting material is sufficiently pulverized and mixed, microscopically uniform distribution cannot be obtained but the size of this grain boundary phase is not uniform, with the result that abnormal growth of sintered grains of silicon nitride is promoted and the formed grain boundary having a large thickness acts as a breaking source. Furthermore, the ion bonding property is increased by addition of the oxide and inherent excellent characteristics of silicon nitride are degraded. Accordingly, in order to improve characteristics of the sintered body, especially mechanical characteristics, it is necessary that a non-oxide type sintering aid should be used and the sintering aid should be uniformly dispersed while reducing the amount added of the sintering aid.
As means for uniformly dispersing a sintering aid, there is ordinarily adopted a method in which the particle size of the starting powder is reduced as much as possible and the dispersion efficiency is increased by using ultra-fine powder. However, if starting powder having such an ultra-fine size is molded, the density of the molded body, that is, the bulk density of a compacted powder body, tends to decrease, and such troubles as reduction of the sintering property, increase of the amount of contraction and degradation of the size precision arise.